1. Field of the Invention
This invention relates to a video clamp for removing a DC offset voltage component from a video signal to simplify video processing circuitry in an image capture data system.
2. Description of the Prior Art
In general, video signals have an AC data component on a DC offset voltage and require a video processing system to continually compensate for these temperature and circuitry dependent DC offset voltages. Prior circuits, developed to eliminate the DC offset voltage from a signal having an AC component before subjecting the signal to processing, have taken various approaches. The Bogdan, Jr., et al patent entitled, Clamp Circuit with a Shunt Unilateral Discharge Path, U.S. Pat. No. 3,159,751, patented Nov. 25, 1960, provides a circuit for periodically referencing a conductor to a predetermined voltage level using a diode configured, unidirectional discharge path. However, the circuit's shortcoming is its dependence upon matched diodes and ideal transistors. Without such components, the DC voltage difference between the input and output will vary with temperature. Also, the coupling capacitor 13, used in clamping the AC signal to the desired DC level, will be unable to maintain its charge due to the discharge paths through connected components having less than virtually infinite input impedence.
The Van Roessel patent, Straylight Compensation Circuit and Blanking Circuit for Same, U.S. Pat. No. 4,001,502, patented Jan. 4, 1977, removes an unwanted DC bias voltage component from a video signal and clamps that signal to a desired DC level for further processing. The patented circuit operates over a wide bandwidth of signals and maintains the chosen amount of DC bias voltage better than prior circuits. Nevertheless, this circuit fails to offer the following advantages embodied in the applicant's invention:
1. The applicant's circuit uses fewer and less expensive components and has better DC voltage stabilization at the chosen level. PA0 2. The applicant offers more freedom for chosing the DC level at which an AC signal is clamped, a better response to transients in the AC data signal, and a newer metal-oxide-semiconductor field effect transistor (MOSFET) technology application which eases implementation constraints.